1. Field of the Invention
The present invention relates to a diffraction type filter used in an optical pickup apparatus which is commonly usable for two or more kinds of optical recording media. In the case where the optical recording media are irradiated with light beams having wavelengths different from each other depending on the kinds of optical recording media, the diffraction type filter can efficiently converge the light beams onto their corresponding optical recording media.
2. Description of the Prior Art
Recently, a variety of optical recording media have been under development, and optical pickup apparatus which can record and reproduce a plurality of kinds of optical recording, media in common have been known.
For example, an optical pickup apparatus may be used for recording and reproducing both DVD (digital video disk) and CD-R (write-once optical disk). In this case, the value of numerical aperture NA for each recording medium is defined according to a standard, such that brightness is necessary for the numerical aperture for DVD, which is 0.6, for example, whereas brightness is suppressed in the numerical aperture for CD-R, which is 0.45, for example.
As a consequence, when an objective lens is to be used for recording and reproducing both of the above-mentioned media, it becomes necessary to switch the numerical aperture of the pickup optical system between their recording/reproducing operations.
Typically, the size of numerical aperture is switched by changing the luminous flux diameter. For example, a liquid crystal shutter may be driven in synchronization with the timing at which the recording/reproducing operations for both media are switched, so as to change the diameter of the luminous flux passing through the shutter.
When a liquid crystal shutter is used for changing the luminous flux diameter, an electric control system is needed for controlling the liquid crystal shutter, thereby increasing the manufacturing cost.
Also, since only a polarized light component having a predetermined direction of vibration can pass through the liquid crystal shutter, it is necessary to carry out designing in view of the direction of vibration of the irradiation light in order for the light to efficiently pass therethrough, whereby the degree of flexibility in designing is restricted.
In the above-mentioned two kinds of optical recording media, while visible light at about 635 nm, for example, is employed for DVD in order to improve its recording density, near infrared light at about 780 nm is needed for CD-R since it has no sensitivity to light in the visible region. As a consequence, an optical pickup apparatus which can commonly be used for both of them would be of a so-called two-wavelength beam system using two different wavelengths of light as the irradiation light. Hence, the luminous flux diameters of these two kinds of irradiation light may be changed by means of their difference in wavelength.
Also, in the case where a pickup optical system is commonly used for optical recording media whose substrates have thicknesses different from each other as in the case of the above-mentioned DVD and CD-R, spherical aberration may occur when an objective lens designed for one of the optical recording media is used for the other optical recording medium, thus making it difficult to record/reproduce signals.